Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises near the interface between the AlGaN barrier layer and the GaN channel layer. In an HEMT, the 2DEG forms the channel of the device. Similar principles may be utilized to select channel and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. Measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor designs. These advantageous conduction characteristics make HEMTs desirable in applications including, but not limited to, use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example.
HEMTs may include so-called back-barrier regions beneath the heterostructure portion, i.e., beneath the channel layer. A back-barrier region can be formed from a type III-V semiconductor having a different bandgap as the channel region, e.g., AlGaN in the case of a GaN channel region. Back-barrier regions are used to increase electron confinement in the channel of the device, and consequently shift the threshold voltage of the device. An example of an HEMT with a back-barrier region is disclosed in U.S. application Ser. No. 15/352,115 to Curatola, the content of which is incorporated by reference herein in its entirety.
Although a back-barrier region can improve the performance of an HEMT, one issue with an HEMT design that includes a back-barrier region is the formation of a secondary intrinsic two-dimensional carrier gas (e.g., a 2DHG) at the interface between the channel and back-barrier regions, due to the difference in bandgap between the materials. This secondary intrinsic two-dimensional carrier gas forms an electrically floating channel in the device that can detrimentally impact device reliability.